90 JOURNAL OF COSMETIC SCIENCE 70 0 black 60 0 ';;?. 50 y= 62.3 -0.28 X .5 40 C 20 y= 35.8 -1.07 X 10L---____..__________.______..._____.__�_.______.______, 0 5 10 15 20 25 30 35 Distance from Root [cm] Figure 4. Means for the gloss index GL at various positions along the hair, given as distance from the hair root for black (0), brown (■), and blonde ( ♦) hair, respectively. The solid lines are the linear regression lines through the data for brown and blonde hair. Filled symbols are used for those cases where the slope of the regression line has a relevant statistical significance. The equations that are given on the graph specifically relate to the individual results. The horizontal line on the right marks the group mean (60.2%) for black hair (see Table I), where the decrease along the hair length is not significant. cells will as a rule not contain pigment. Light is hence refracted into the cuticle and passes through a substantial number of cuticle cells with reflection, refraction, and scattering at the cell interfaces until ultimate reflection at the cuticle/cortex interface generates a major component of what is observed as diffusely reflected light. This effect will further be intensified if, through hair damage, delamination has occurred between cuticle cell layers (19,23) or if, more generally, cracks, voids, or inclusions have been formed. The gloss measurement of hair, which due to its color does not allow light reflection from the fiber interior, thus provides through the diffuse component of reflected light information on the structural status of its cuticle layer. In this context the differences in the change of gloss along the hair length for the black and the brown hair (see Figure 4) are interesting to note. It is important to note that the approach, described in this paper, for determining the gloss index is equally applicable for African hair, despite its strong ellipticity and curliness. However, this type of hair was not included in the study, since it was considered as being less suitable in the context of determining the effects of hair color and along-hair changes.
MEASURE OF HAIR LUSTER 91 In practice, hair shine is a very complex phenomenon (16,24), which involves physical, physiological, and psychophysical considerations. Gv on the other hand, provides an absolute, numerical scale. In this view, it has to be noted that consumer perception cannot be expected to follow an equisense scale in which a unit of difference indicates an equal sensory difference in luster (3). In practice, an expert or a consumer will generally rank product performance in a contrast situation (half-head test, before-vs-after, product A vs product B, etc.) relating to relative changes of appearance ( 4), rather than work on the basis of ordinal sensory units. As was shown in small scale panel tests (18,25), these rank differences appear to be closely related to the laboratory situation of gloss testing of single hair fibers described here. However, a somewhat more comprehensive argument for the feasibility of the gloss index to model the assessment of hair luster by a consumer panel is derived from the extensive investigations by Reich and Robbins (11). They found a linear relationship between the inverse of the width of the GP curve at half height and the luster ranking by panels. Applying the data given in Part I (1) for the pos1t10ns and widths of the Gaussian distributions describing specular and diffuse reflection, theoretical GP curves were calculated on the basis of equation 3, assuming the presence of Sand D5 only, for a wide range of GL values. Finally, the width at half-height w of the theoretical GP curves was 12--,....._.........,--,---.,---.,-,.---.-----------.------------ C' 10 "ii 8 ::c (;f d 1a .c , p J -i ' 0 4 C, Q T'" , 0 2-----------------......----------"' 0 20 40 60 80 100 Gloss Index In % Figure 5. Inverse of the width at half-height for calculated GP curves on the basis of a presumed value for the gloss index. The largely linear relationship for GL values beyond 25% is indicated by the straight line.
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